To date, 18 HDACs have been identified in humans and there is increasing evidence that the 18 histone deacetylases (HDAC) in humans are not redundant in function. HDACs are classified into three main groups based on their homology to yeast proteins. Class I includes HDAC1, HDAC2, HDAC3, and HDAC8 and have homology to yeast RPD3. HDAC4, HDAC5, HDAC7, and HDAC9 belong to class IIa and have homology to yeast HDAC1. HDAC6 and HDAC10 contain two catalytic sites and are classified as class IIb, whereas HDAC11 has conserved residues in its catalytic center that are shared by both class I and class II deacetylases and is placed in class IV. These HDACs contain zinc in their catalytic site and are inhibited by compounds like trichostatin A (TSA) and vorinostat [suberoylanilide hydroxamic acid (SAHA)]. Class III HDACs are known as sirtuins. They have homology to yeast Sir2, require NAD+ as cofactor, and do not contain zinc in the catalytic site. In general, HDAC inhibitors of zinc-dependent HDACs include a Zn-binding group, as well as a surface recognition domain.
HDACs are involved in the regulation of a number of cellular processes. Histone acetyltransferases (HATs) and HDACs acetylate and deacetylate lysine residues on the N termini of histone proteins thereby affecting transcriptional activity. They have also been shown to regulate post-translational acetylation of at least 50 non-histone proteins such as α-tubulin (see for example Kahn, N et al Biochem J 409 (2008) 581, Dokmanovic, M et al Mol Cancer Res 5 (2007) 981).
Altering gene expression through chromatin modification can be accomplished by inhibiting histone deacetylase (HDAC) enzymes. There is evidence that histone acetylation and deacetylation are mechanisms by which transcriptional regulation in a cell—a major event in cell differentiation, proliferation, and apoptosis—is achieved. It has been hypothesized that these effects occur through changes in the structure of chromatin by altering the affinity of histone proteins for coiled DNA in the nucleosome. Hypoacetylation of histone proteins is believed to increase the interaction of the histone with the DNA phosphate backbone. Tighter binding between the histone protein and DNA can render the DNA inaccessible to transcriptional regulatory elements and machinery. HDACs have been shown to catalyze the removal of acetyl groups from the ϵ-amino groups of lysine residues present within the N-terminal extension of core histones, thereby leading to hypoacetylation of the histones and blocking of the transcriptional machinery and regulatory elements.
Inhibition of HDAC, therefore can lead to histone deacetylase-mediated transcriptional derepression of tumor suppressor genes. For example, cells treated in culture with HDAC inhibitors have shown a consistent induction of the kinase inhibitor p21, which plays an important role in cell cycle arrest. HDAC inhibitors are thought to increase the rate of transcription of p21 by propagating the hyperacetylated state of histones in the region of the p21 gene, thereby making the gene accessible to transcriptional machinery. Further, non-histone proteins involved in the regulation of cell death and cell-cycle also undergo lysine acetylation and deacetylation by HDACs and histone acetyl transferase (HATs).
This evidence supports the use of HDAC inhibitors in treating various types of cancers. For example, vorinostat (suberoylanilide hydroxamic acid (SAHA)) has been approved by the FDA to treat cutaneous T-cell lymphoma and is being investigated for the treatment of solid and hematological tumors. Further, other HDAC inhibitors are in development for the treatment of acute myelogenous leukemia, Hodgkin's disease, myelodysplastic syndromes and solid tumor cancers.
HDAC inhibitors have also been shown to inhibit pro-inflammatory cytokines, such as those involved in autoimmune and inflammatory disorders (e.g. TNF-α). For example, the HDAC inhibitor MS275 was shown to slow disease progression and joint destruction in collagen-induced arthritis in rat and mouse models. Other HDAC inhibitors have been shown to have efficacy in treating or ameliorating inflammatory disorders or conditions in in vivo models or tests for disorders such as Crohn's disease, colitis, and airway inflammation and hyper-responsiveness. HDAC inhibitors have also been shown to ameliorate spinal cord inflammation, demyelination, and neuronal and axonal loss in experimental autoimmune encephalomyelitis (see for example Wanf L et al, Nat Rev Drug Disc 8 (2009) 969).
Triplet repeat expansion in genomic DNA is associated with many neurological conditions (e.g., neurodegenerative and neuromuscular diseases) including myotonic dystrophy, spinal muscular atrophy, fragile X syndrome, Huntington's disease, spinocerebellar ataxias, amyotrophic lateral sclerosis, Kennedy's disease, spinal and bulbar muscular atrophy, Friedreich's ataxia and Alzheimer's disease. Triplet repeat expansion may cause disease by altering gene expression. For example, in Huntington's disease, spinocerebellar ataxias, fragile X syndrome, and myotonic dystrophy, expanded repeats lead to gene silencing. In Friedreich's ataxia, the DNA abnormality found in 98% of FRDA patients is an unstable hyper-expansion of a GAA triplet repeat in the first intron of the frataxin gene (see Campuzano et al., Science 271:1423 (1996)), which leads to frataxin insufficiency resulting in a progressive spinocerebellar neurodegeneration. Since they can affect transcription and potentially correct transcriptional dysregulation, HDAC inhibitors have been tested and have been shown to positively affect neurodegenerative diseases (see Herman D et al, Nat Chem Bio 2 551 (2006) for Friedreich's ataxia, Thomas E A et al, Proc Natl Acad Sci USA 105 15564 (2008) for Huntington's disease).
HDAC inhibitors may also play a role in cognition-related conditions and diseases. It has indeed become increasingly evident that transcription is likely a key element for long-term memory processes (Alberini C M, Physiol Rev 89 121 (2009)) thus highlighting another role for CNS-penetrant HDAC inhibitors. Although studies have shown that treatment with non-specific HDAC inhibitors such as sodium butyrate can lead to long-term memory formation (Stefanko D P et al, Proc Natl Acad Sci USA 106 9447 (2009)), little is known about the role of specific isoforms. A limited number of studies have shown that, within class I HDACs, main target of sodium butyrate, the prototypical inhibitor used in cognition studies, HDAC2 (Guan J-S et al, Nature 459 55 (2009)) and HDAC3 (McQuown S C et al, J Neurosci 31 764 (2011)) have been shown to regulate memory processes and as such are interesting targets for memory enhancement or extinction in memory-affecting conditions such as, but not limited to, Alzheimer's disease, post-traumatic stress disorder or drug addiction.